Let's look at the definitions of the tiers:
C Rank: Reserved for Pokemon who perform specific niches and can be effective given the right support, but either are incapable of performing well outside of that niche or are heavily dependent on that support.
D Rank: Reserved for Pokemon who are simply not very effective in the current metagame.
Sableye does not fit into D rank. It isn't "not very effective in the current metagame". As many have stated previously, Sableye is an efficient annoyer and check to many physical threats; it supports both stall and offense teams with its unique combination of typing, (good enough) bulk, movepool, and effective speed. It is in the group of Pokemon who "perform specific niches and can be effective given the right support, but either are incapable of performing well outside of that niche or are heavily dependent on that support." Sableye does indeed effectively perform a specific niche. Its ability to repeatedly check and cripple Extremekiller Arceus is practically unparallelled; while both Skarmory and Giratina can check it fairly well, Skarmory lacks the ability to do anything but force it out and Giratina falls to boosted Ghost-type attacks with residual damage. You mention that it lacks the ability to take special attacks from the tier, as well as the fact that Will-O-Wisp is unreliable. While I wholeheartedly agree that these flaws are apparent, I encourage you to take a look at some of the other members of C.
All of these Pokemon have either crippling flaws or are generally outclassed. They all require significant support to function at their best, but they have a place in the tier as a threat. They all impose a significant risk on the teambuilder, a risk that if used in the wrong situation (or even if the odds aren't "ever in their favor") they can become liabilities. Sableye fits quite well into this group; while it is quite effective at what it does, it can become a liability against almost every special attacker in the tier, allowing for a turn of setup due to its inherent weakness against such pokes.
"May the odds be ever in your favor"
"WTF HAX NOOB"
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